Have you tried rebuilding mailboxes? How about using an application like OnyX or Leopard Cache Cleaner to clear the system caches?
Currently using an iMac running 10.4.11. About a month ago mail started acting funny. On some incoming mail only part of the graphics would open. When I double clicked, the complete message and/or graphics open. No one at the Apple Store has any idea why this would happen. Any thoughts out there?

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